Method, mobile station and base station for frequency synchronization of a mobile station in a radio communications system

ABSTRACT

A TDMA/CDMA radio communications system provides frequency channels which are formed both by time slots and by broadband frequency ranges, and in which information from a plurality of connections can be simultaneously transmitted between mobile stations and a base station wherein it is possible to distinguish the information from different connections in accordance with a connection-specific fine structure. Frequency channels are repeatedly provided for frequency synchronization of the mobile stations in the downward direction in which a symbol sequence is transmitted. From received signals, the mobile station to be synchronized determines estimated values for the symbol sequence and compares these estimated values with a reference sequence. It is thereby possible to calculate a phase drift in the estimated values with reference to the reference sequence wherein a frequency offset, which is used for frequency synchronization, is determined from the phase drift.

The present invention relates generally to a method for frequencysynchronization of a mobile station in a radio communications systems(in particular, a TDMA/CDMA radio communications system), and, morespecifically to a mobile station and to a base station which can carryout such method.

DESCRIPTION OF THE PRIOR ART

The design of digital radio communications systems is shown in J.Oudelaar, “Evolution towards UMTS”, PIMRC 94, 5th IEEE InternationalSymp. on Personal, Indoor and Mobile Radio Communications, The Hague,NL, Sep. 18-22 1994, pages 852-856, and M. Lenti, H. Hageman, “Paging inUMTS”, RACE Mobile Telecommunications Workshop, Vol. 1, Amsterdam, NL,May 17-19 1994, pages 405-410.

The presently known mobile radio system GSM (Global System for MobileCommunications) is a radio communications system with a TDMA componentfor subscriber separation (time division multiple access). Userinformation from the subscriber connections is transmitted in time slotsin accordance with a frame structure. The transmission is performed inblocks. Furthermore, frequency channels (FCCH frequency correctionchannels) matched to the timing pattern of the frame structure andserving the purpose of frequency synchronization for the mobile stationsare known in the downward direction from the GSM mobile radio systems.In this frequency channel, a mobile station can evaluate a sinusoidalcarrier for the purpose of self-synchronization. Frequencysynchronization by means of a sinusoidal carrier is disclosed, forexample, in WO 91 10305 A.

Time synchronization of time slots in a TDMA radio communications systemis disclosed in EP 0 318 684. D1 discloses a method for timesynchronization of a mobile station. The radio transmission channel hasa signal delay between the base station and mobile station which isproportional to the distance between the mobile station and the basestation. In addition, the signal delay can vary strongly in time becauseof reflections. The signals received by the mobile station are delayedtemporally with different severity by the signal delay. This delaying ofthe signal between the transmitted signal and the received signal isdenoted as phase error. The compensation of this delaying of the signalis the subject matter of the method for time synchronization. In suchcase, the delaying of the signal is constant within one TDMA time slot.

DE 195 49 148.3 discloses a mobile communications system which uses aTDMA/CDMA subscriber separation (CDMA code division multiple access),and applies at the receiving end a JD method (joint detection) in orderto undertaken with knowledge of sequence spread codes of a plurality ofsubscribers to, improved detection of the transmitted user information.Information from a plurality of user data connections which can bedistinguished by their spread code are transmitted simultaneously in onefrequency channel (TCH traffic channel). However, the division ofspecific frequency channels for synchronization purposes results in alarge loss in capacity in comparison with the GSM system since thefrequency range used for a frequency channel is a more broadband one.

It is an object of the present invention, therefore, to provide a methodand devices which permit frequency synchronization in conjunction withlow consumption of radio resources in a radio communications system.

SUMMARY OF THE INVENTION

A radio communications system provides frequency channels which areformed by time slots and by broadband frequency ranges, and in whichinformation from several connections is transmitted simultaneouslybetween mobile stations and a base station; it being possible todistinguish the information from different connections in accordancewith a connection-specific fine structure. The different connectionsalso can be formed by a plurality of codes which are assigned to asingle mobile station.

According to the present invention, frequency channels in which a symbolsequence is transmitted are temporally repeatedly provided for frequencysynchronization for the mobile stations in the downward direction. Fromreceived signals, the mobile station to be synchronized determinesestimated values for the symbol sequence and compares them with areference sequence. Consequently, a phase drift in the estimated valueswith reference to the reference sequence can be calculated. A frequencyoffset to which is used for frequency synchronization is determined fromthe phase drift.

By virtue of the fact that a phase drift can be calculated in accordancewith the comparison, the possibility arises of simply determining thefrequency offset of the mobile station to be synchronized with referenceto the carrier of the frequency channel. A sufficiently large andappropriately distributed number of samples is sufficient for thecomparison. It is thereby possible for the frequency synchronization tobe combined with other measures for synchronizing the mobile station sothat the use of radio resources remains low.

Advantageously, the symbol sequence is transmitted in addition toinformation from further connections. This also means that the radioresources of the air interface between the base station and mobilestations can be better used. Because information can be distinguished inaccordance with an impressed fine structure, a time slot is not blockedsolely by the synchronization. Rather, such time slot can be usedmultifariously. The further connections are, in this case, user dataconnections or signaling connections.

In an embodiment of the present invention, the symbol sequence istransmitted in such a way that at least two estimated values areobtained at a temporal spacing sufficiently large for the phase driftcalculation. It is, therefore, not necessary for the symbol sequence tobe a continuous sequence. There can be individual symbols or groups ofsymbols within further known or unknown symbols to be transmitted. Thesymbols can last over the time period of a time slot, or they can betransmitted in addition to training sequences and/or symbols for othertuning purposes.

The symbol sequence is advantageously spread with the aid of anindividual spread code, it being possible for the information on thepresence of a symbol sequence to be included in the spread code for timesynchronization. The symbol sequence also can be interpreted as asequence of chips which generate a bandwidth which fills up thefrequency range. An additional outlay on processing at the receiver iseliminated by virtue of the fact that the message block with the datafor time synchronization can be processed together with the remainingmessage blocks by despreading.

In accordance with a further embodiment of the present invention, thephase drift is calculated as a proportionality factor in accordance withthe method of least error squares from the comparison of the estimatedvalues with the reference sequence. A linear relationship is presupposedbetween the estimated values and reference sequence; that is, anapproximately constant frequency offset. This obtains in the case of thehigh accuracies of the frequency normal in the mobile station.

It is advantageous to use at least parts of the symbol sequence both forthe time synchronization and for the frequency synchronization. Thus, itis possible with the aid of a symbol sequence, for example bycorrelation, to produce the temporal reference of the transmissionwithin a time slot and, in addition, the frequency reference bydetermining the phase drift. Only a low network capacity need thereforebe set aside for synchronization.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic block diagram of a mobile radio network;

FIG. 2 shows a schematic representation of the frame structure of aradio transmission in the mobile radio network;

FIG. 3 shows a schematic representation of the structure of a frequencychannel for time synchronization; and

FIG. 4 shows block diagrams of mobile stations and a base station withradio transmission in a downward direction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The radio communications system represented in FIG. 1 corresponds instructure to a known GSM mobile radio network which includes amultiplicity of mobile switching centers MSC which are networked withone another and which provide the access to a landline network PSTN.Furthermore, these mobile switching centers MSC are respectivelyconnected to at least one base station controller BSC. Each base stationcontroller BSC, in turn, permits connection to at least one base stationBS. Such a base station BS is a radio station which can establish amessage connection to mobile stations MS via an air interface.

By way of example, FIG. 1 represents three connections for transmittingboth user information and signaling information between three mobilestations MS and a base station BS. An operating and maintenance centerOMC implements control functions and maintenance functions for themobile radio network, or for parts thereof. The functionality of thisstructure can be transferred to other radio communications systems inwhich the present invention can be used.

The frame structure of the radio transmission can be seen in FIG. 2.Partitioning of a broadband frequency range (for example, the bandwidthB=1.6 MHz) into a plurality of time slots ts (for example, 8 time slotsts1 to ts8) is provided in accordance with a TDMA component. Each timeslot ts within the frequency range B forms a frequency channel FK.Within the frequency channels TCH, which alone are provided fortransmitting user data, information from a plurality of connections istransmitted in message blocks.

These message blocks for transmitting user data comprise sections withdata d in which sections with training sequences tseq1 to tseqK known atthe receiving end are embedded. The data d is spread in aconnection-specific fashion with a fine structure (a subscriber code c)with the result that at the receiving end K connections can be separatedby these CDMA components, for example.

The spreading of individual symbols of the data d has the effect that Qchips of duration T_(chip) are transmitted within the symbol durationT_(sym). The Q chips in this case form the connection-specificsubscriber code c. Furthermore, a protection time gp for compensatingdifferent signal delays of the connections is provided within the timeslot ts.

Within a broadband frequency range B, the sequential time slots ts aresubdivided in accordance with a frame structure. Thus, eight time slotsts are combined to form a frame wherein, for example, one time slot ofthe frame forms a frequency channel TCH for user data transmission andis repeatedly used by a group of connections. A frequency channel FCCHfor frequency synchronization of the mobile stations MS is not insertedin every frame, but rather at a prescribed instant within a multiframe.The spacings between the frequency channels FCCH for frequencysynchronization determine the capacity made available therefor by themobile radio network.

The structure of a frequency channel FCCH for frequency synchronizationis shown with the aid of FIG. 3. The broadband frequency range B in thefrequency band of an organization channel of the corresponding cell ofthe mobile radio network is part of a frame R0. Such frame R0 includes afrequency channel FCCH for frequency synchronization (in which, however,further signaling and user data connections are also handled) and, inthe following time slots, exclusive frequency channels TCH for user datatransmission or signaling transmission.

This frame R0 is, in turn, part of a superframe S0 which, in addition tothe frame R0, includes both a frame R1 having frequency channels FK withfurther cell-related information and a frame R2 with user data. Ahyperframe includes, in turn, a plurality of superframes S0, S1, ofwhich at least one contains a frequency channel FCCH for frequencysynchronization.

Transmitted in the downward direction in a time slot ts of the frequencychannel FCCH for frequency synchronization is a symbol sequence f1 whichis known in the mobile stations MS as a reference sequence. The symbolsequence f1 is spread with the aid of an individual code c1.

An alternative embodiment for the symbol sequence f1 provides that partsof the symbol sequence f1 are used as a synchronization block sb fortime synchronization. This synchronization block sb is arranged in themiddle of the time slot ts. The base station BS radiates the symbolsequence f1 with a transmitter power which is matched for the purpose ofpower control to the received power of the remaining connections.

At the receiving end, an evaluation of the information transmitted inthe downward direction is carried out for frequency synchronization.FIG. 4 shows the radio transmission in the downward direction from thebase station BS to mobile stations MS1 to MSK. The mobile stations MSfirstly determine one or more frequency ranges B with a sufficientlyhigh or maximum received power. These are, in general, the frequencyranges B of the nearest base station BS in whose cell the mobile stationMS is instantaneously located.

Mobile stations MS1 to MSK evaluate received signals in these frequencyranges B and carry out continuous correlation of the values of therespective received signals with the reference sequence f2. Given asufficiently large correlation, the specific instant t1 of the arrivalof the symbol sequence f1 is selected as reference point for timesynchronization and an internal time reference of the mobile station MSis tuned.

Estimated values f1′ are simultaneously determined for the symbolsequence f1 from the received signals. Thereupon, a system of equationsis set up which compares the estimated values f1′ with the values of thereference sequence f2 which corresponds to the symbol sequence f1. Aproportionality factor which represents a phase drift dp in theestimated values f1′ with reference to the reference sequence f2 isdetermined by a calculation with the aid of the method of least errorsquares, or of another method of solution.

A frequency offset df is derived from the phase drift dp in accordancewith the relationship:

dp=e ^(j2π·df·t),

t representing time.

This frequency offset df of the internal frequency, which is normalrelative to the carrier frequency of the frequency range B of thefrequency channel FCCH for frequency synchronization, is applied to theinternal frequency normal to the correct sign. The frequencysynchronization is thereby carried out.

The base station BS includes a transceiver SE/EE which subjects thetransmitted signals which are to be emitted to digital-to-analogconversion, converts from the baseband into the frequency range B of theemission, and modulates and amplifies the transmitted signals. A signalgenerating device SA has previously assembled the transmitted signals,for example the symbol sequence f1, and assigned them to thecorresponding frequency channels FCCH, TCH.

The mobile station MS contains an operating panel T, a signal processingdevice SP, a control device SE and a transceiver SE/EE. On the operatingpanel T, the subscriber can make inputs, including an input foractivating the mobile station MS, which thereupon must first carry outsynchronization with the mobile radio network surrounding it.

The control device SE then receives this request and causes the signalprocessing device SP to evaluate signals received via the transceiverSE/EE such that, as already described, the appropriate frequency range Bis selected and correlation is carried out until the symbol sequence f1is successfully found. The time and frequency synchronization describedis subsequently carried out.

For the purpose of signal processing, the received signals are convertedinto symbols with a discrete store of values; for example, digitized.This signal processing device SP, which as a digital signal processorcontains a JD processor for detecting both the user information and thesignaling information using the JD-CMDA method (joint detection), alsoevaluates the symbol sequence f1. Although the present invention hasbeen described with reference to specific embodiments, those of skill inthe art will recognize that changes may be made thereto withoutdeparting from the spirit and scope of the invention as set forth in thehereafter appended claims.

We claim:
 1. A method for frequency synchronization of a mobile stationof a radio communications system, the method comprising the steps of:providing frequency channels in the radio communications system whichare formed both by time slots and by broadband frequency ranges whereininformation from one or more connections is transmitted simultaneouslyin the frequency channels between mobile stations and base stations, andwherein the information from different connections is distinguishable byspreading codes; providing one of the frequency channels for repeatedfrequency synchronization of the mobile stations in a downwarddirection; transmitting a chip sequence within one frequency channel;determining estimated values for the chip sequence from received signalsby the mobile station to be synchronized; calculating a phase drift inthe estimated values from at least a comparison of the estimated valueswith a reference chip sequence, which is identical to the transmittedchip sequence; determining a frequency offset from the phase drift; andusing the frequency offset by the mobile station for frequencysynchronization.
 2. A method for frequency synchronization of a mobilestation of a radio communications system as claimed in claim 1, whereinthe step of transmitting a chip sequence further comprisessimultaneously transmitting the chip sequence with information to betransmitted from at least one of user data connections and signalingconnections.
 3. A method for frequency. synchronization of a mobilestation of a radio communications system as claimed in claim 1, whereinthe step of transmitting a chip sequence further comprises obtaining atleast two estimated values for calculating the phase drift.
 4. A methodfor frequency synchronization of a mobile station of a radiocommunications system as claimed in claim 1, further comprising the stepof: spreading the chip sequence with the aid of an individual spreadcode.
 5. A method for frequency synchronization of a mobile station of aradio communications system as claimed in claim 1, wherein the phasedrift is calculated as a proportionality factor in accordance with amethod of least error squares from the comparison of the estimatedvalues with the reference sequence.
 6. A method for frequencysynchronization of a mobile station of a radio communications system asclaimed in claim 1, further comprising the step of: using at least partof the chip sequence for time synchronization and frequencysynchronization.
 7. A method for frequency synchronization of a mobilestation of a radio communications system as claimed in claim 1, furthercomprising the step of: separating the signals in a frequency channelusing a JD-CDMA method.
 8. A method for frequency synchronization of amobile station of a radio communications system as claimed in claim 1,further comprising the step of: selecting the frequency range with thefrequency channel for frequency synchronization by the mobile station inaccordance with a measurement of receiver power.
 9. A mobile station ofa radio communications system for use in connection with a method forfrequency synchronization of the mobile station, wherein the radiocommunications system provides frequency channels which are formed bothby time slots and by broadband frequency ranges and in which informationfrom one or more connections is simultaneously transmitted betweenmobile stations and base stations, wherein information from differentconnections is distinguishable by spreading codes, wherein one of thefrequency channels is provided for repeated frequency synchronization ofthe mobile stations in a downward direction, and wherein a chip sequenceis transmitted within the one frequency channel, the mobile stationcomprising: a signal processing device for determining estimated valuesfor the chip sequence from received signals, for comparing the estimatedvalues with a reference chip sequence, which is identical to thetransmitted chip sequence and for calculating a frequency offset fromthe comparison; and a control device for frequency synchronization whichtakes into account the frequency offset.
 10. A base station for use inconnection with a method for frequency synchronization of a mobilestation of a radio communications system, wherein the radiocommunications system provides frequency channels which are formed bothby time slots and by broadband frequency ranges and in which informationfrom one or more connections is simultaneously transmitted betweenmobile stations and base stations, wherein the information fromdifferent connections is distinguishable by spreading codes, and whereinone of the frequency channels is provided for repeated frequencysynchronization of the mobile stations in a downward direction, the basestation comprising: a signal generating device for generating apredetermined chip sequence; and a transmitting device for transmittingthe chip sequence in the one frequency channel for frequencysynchronization, wherein estimated values for the chip sequence aredetermined from received signals by the mobile station to besynchronized, a phase drift in the estimated values is calculated fromat least a comparison of the estimated values with a reference chipsequence which is identical to the transmitted chip sequence, afrequency offset is determined from the phase drift, and the frequencyoffset is used by the mobile station for frequency synchronization.